Sensory system for improved balance control

ABSTRACT

A feedback device for measuring balance related information, and for producing a stimulation of the skin that encodes that information in a way that is useful to the wearer of the device. At least one sensor detects balance information and transmits at least one balance information signal to a signal processing subsystem. The signal processing subsystem converts the received balance information signal into at least one stimulation control signal. The signal processing subsystem then transmits the stimulation control signal to at least one stimulator, which provides stimulation to a wearer of the device reflecting the stimulation control signal received from the signal processing subsystem.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority as a continuation of U.S. patentapplication Ser. No. 15/052,349, entitled “Sensory Prosthetic forImproved Balance Control” and filed Feb. 24, 2016; which claims priorityas a continuation of U.S. Pat. No. 9,289,174, entitled “SensoryProsthetic for Improved Balance Control,” which issued on Mar. 22, 2016;which claims priority as a continuation of U.S. Pat. No. 9,402,580,entitled “Sensory Prosthetic for Improved Balance Control,” which issuedon Aug. 2, 2016, which claims priority as a continuation of U.S. Pat.No. 8,974,402, entitled “Sensory Prosthetic for Improved BalanceControl,” which issued on Mar. 10, 2015, which claims priority to U.S.Patent Application 60/372,148, entitled “Sensor Prosthetic for ImprovedBalance Control” and filed Apr. 12, 2002, all of which are herebyincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to systems for improving balancecontrol, and more specifically to a feedback device which measuresbalance related information, and produces a stimulation of the skin thatencodes that information.

BACKGROUND OF THE INVENTION

It has been estimated that as much as 20% of the elderly population inthe United States may be suffering from peripheral neuropathies, largelyas a consequence of diabetes. Peripheral neuropathic patients exhibitincreased body sway during quiet standing. Peripheral neuropathies havebeen associated with increased thresholds for the perception of ankleinversion/eversion and a reduced ability to maintain a unipedal (singlefooted) stance, suggesting a reduction in balance control while walking.Epidemiological evidence has linked peripheral neuropathies with anincreased risk of falling. Postural responses to floor perturbations inperipheral (diabetic) neuropathy patients are delayed and are poorlyscaled to the perturbation amplitude

The most common symptom of peripheral neuropathies is a reduction insensation from the soles of the feet. A number of studies have providedevidence that afferent information from the feet is an important part ofthe balance control system. A recent study on adaptationto microgravitysuggests that foot sole pressure may be critical for triggering theanticipatory postural adjustments that are normally required to maintainbalance during arm movements.

For the above reasons and others, it would be desirable to have asensory substitution system that effectively provides informationregarding foot sole pressure distribution patients who are no longerable to acquire this information by natural means. The system shouldenable a patient wearing a device to achieve improved upright balancecontrol, thereby reducing the patient's risk of falls and associatedinjuries. Such a system should further advantageously supportintegration of balance related feedback into the patient's unconsciouspostural control system, eventually eliminating the need for consciouseffort in this regard.

BRIEF SUMMARY OF THE INVENTION

A feedback device is disclosed for measuring balance relatedinformation, and for producing a stimulation of the skin that encodesthat information in a way that is useful to the wearer of the device.The disclosed device consists of at least one sensor for detectingbalance information and for transmitting at least one balanceinformation signal to a signal processing subsystem. The signalprocessing subsystem converts the received balance information signalinto at least one stimulation control signal. The signal processingsubsystem then transmits at least one stimulation control signal to atleast one stimulator, which provides stimulation to a wearer of thedevice reflecting the stimulation control signal received from thesignal processing subsystem.

In one embodiment, an array of sensors are arranged under the soles ofeach foot of the wearer. The sensors operate to transduce the magnitudeof pressure exerted on the foot sole at each sensor location into abalance information signal. A signal processing subsystem operates toconvert the balance information signals obtained from the sensors intoestimates of the location and magnitude of the resultant ground reactionforce exerted on each foot, generally referred to as center-of-pressure,or “COP”. The signal processing subsystem then encodes the estimated COPinto stimulation control signals that drive elements of a stimulatorarray. Further in such an embodiment, the stimulator is made up of anarray of vibrotactile stimulators for placement on the user's leg in oneor more planes (also referred to as on the user's leg in one or moreplanes (also referred to as vibrator “levels”) approximately parallel tothe plane of the foot sole. Stimulators are arranged within each planecorresponding to at least four locations on each leg: anterior,posterior, medial,& lateral. In response to signals produced by thesignal processing subsystem, the stimulator array provides vibrotactilestimulation of the skin of the leg representing the estimated COP. Insuch an embodiment, the disclosed system provides a portable, wearabledevice, by which the subject receives cutaneous stimulation on the legregarding the location and magnitude of the ground reaction force underthe ipsilateral foot. With training, a patient suffering from reducedplantar sensation will learn to make postural corrections in response tothis stimulation in the same manner as a healthy person would react tochanges in the pressure distribution under their feet.

Thus there is disclosed a sensory substitution system for providinginformation regarding foot sole pressure distribution to users who areno longer able to acquire this information by natural means. A userwearing this device will achieve improved upright balance control,reducing their risk of falls and associated injuries. With practice,this encoded balance information provided by the device may beintegrated into a patients unconscious postural control system,eliminating the need for conscious balancing effort. The disclosedsystem may also be embodied so as to reduce the balance deficits causedby prolonged exposure to reduced weight bearing, as seen in patientsrecovering from prolonged bed rest or in astronauts returning toterrestrial gravity. Preventative treatments with the disclosed devicemay also reduce the hypersensitivity of foot soles in some users, whichwould otherwise contribute to postural deficits.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing detailed description of the invention in conjunction with thedrawings, of which:

FIG. 1 shows the structure and operation of an illustrative embodimentof the disclosed system;

FIG. 2 shows a circuit diagram of a portion of an illustrativeembodiment of the disclosed system;

FIG. 3 is a flow chart showing steps performed during operation of anillustrative embodiment of the disclosed system;

FIG. 4 shows an illustrative embodiment of the disclosed system using asingle level stimulation array;

FIG. 5 shows an illustrative embodiment of the disclosed system using amulti-level stimulation array;

FIG. 6 shows an illustrative embodiment of the disclosed system using amulti-level stimulation array in combination with a bi-axial goniometer;

FIG. 7 illustrates an example of force magnitude encoding in amultilevel stimulator embodiment; and

FIG. 8-12 show the mapping of balance information to stimulation pointsin an illustrative embodiment.

DETAILED DESCRIPTION

All disclosures of provisional patent application Ser. No. 60/372,148entitled “SENSORY PROSTHETIC FOR IMPROVED BALANCE CONTROL”, and filedApr. 12, 2002, are hereby incorporated by reference herein.

FIG. 1 shows the structure and operation of an illustrative embodimentof the disclosed system. As shown in FIG. 1, sensors 10 detect balanceinformation that may represent foot force/pressure, such as normaland/or shear pressure, ankle angle, such as sagittal and/or coronalankle angle, or knee angle, such as sagittal knee angle. The balanceinformation detected by the sensors 10 is passed to a signal processingsubsystem 12, which converts the balance information received from thesensors 10 into a stimulation control signal that is passed to thestimulators 14. The stimulators 14 operate in response to stimulationcontrol signal, producing a stimuli to a user of the disclosed systemrepresenting the balance information detected by the sensors 10. Thestimulators 14 may be of various types, including external and/orinternal stimulators, and provide various types of stimuli, includingvibratory, visual, auditory, and/or electrical stimuli. The stimulators14 may be located on various parts of the body, including one or bothlegs, head, arms or trunk of the user.

FIG. 2 shows a circuit diagram of a portion of an illustrativeembodiment of the disclosed system. As shown in FIG. 2, a number of footpressure sensors 16 transmit at least one balance information signal toone or more amplifiers 20 within a signal processing subsystem 18. Theamplifiers 20 may, for example, include one or more LM324 analogamplifiers supplied by National Semiconductor Corporation. The amplifiedsignals output from the amplifiers 20 are passed to a signal processor22. In the example of FIG. 2, the signal processor includes analog todigital processing functionality, as well as program code storage forcode executable on the signal processor 22. The signal processor 22 may,for example, consist of a PIC 16F877 microcontroller, supplied byMicrochip Technology, Inc. The signal processor 22 passes a digitalstimulation control signal to one or more digital to analog converters24, which convert the digital stimulation control signal to an analogstimulation control signal that is passed to one or more vibratoryfeedback devices 26. The vibratory feedback devices 26 provide avibratory stimulus to a user wearing the vibratory feedback devices 26,such that the vibratory stimulus encodes and/or represents the balanceinformation provided from the foot pressure sensors 16 to the signalprocessing subsystem 18. The digital to analog converters 24 may, forexample, include one or more TLC72261N, 8 bit, 4 channel digital toanalog converters. The vibratory feedback devices may, for example,include one or more analog amplifiers, such as AD8534 analog amplifiers,supplied by Analog Devices, Inc., as well as a number of vibrators.

FIG. 3 is a flow chart showing steps performed during operation of anillustrative embodiment of the disclosed system. The steps shown in FIG.3 may, for example be performed at least in part by the signalprocessing subsystem 18 shown in FIG. 2, for exampleunder control of anassembly language program stored within and executed on the signalprocessor 22 shown in FIG. 2. As shown in FIG. 3, at step 28, thedisclosed system reads balance information signals output from one ormore sensors, and stores, the values for subsequent processing. Thedisclosed system then operates to convert the stored sensor values intostimulation control values at step 30. The stimulation control valuesare then output at step 32.

An example of the processing performed by the signal processingsubsystem to convert sensor values into stimulation control values isnow provided. A formula describing the steps performed by the signalprocessing subsystem to convert sensor values into stimulation controlvalues is as follows:

$\begin{matrix}{{{\Theta_{l}(t)} = {\sum\limits_{n = 1}^{N}\;{{f_{nl}(t)}\theta_{nl}}}},{{R_{l}(t)} = {\sum\limits_{n = 1}^{N}\;{{f_{nl}(t)}r_{nl}}}},{{F_{l}(t)} = {\frac{1}{W}{\sum\limits_{n = 1}^{N}\;{f_{nl}(t)}}}},} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

where

f_(nl)=normal force, approximately perpendicular to the plane of thefoot sole, measured by sensor array element n under leg 1,

θ_(nl)=angular position of sensor array element n under leg 1,

r_(nl)=radial position of sensor array element n under leg 1,

N=total number of sensor array elements under leg 1,

W=total body weight of the wearer,

Θ_(l)=angular position of center-of-pressure under leg 1,

R_(l)=radial position of center-of-pressure under leg 1,

F_(l)=portion of body weight supported by leg 1, and,

t=a variable representing discrete time

$\begin{matrix}{{{Q_{\Theta\; l}(t)} = {{P_{\Theta}{\Theta_{l}(t)}} + {D_{\Theta}\frac{d\;{\Theta_{l}(t)}}{d\; t}} + {I_{\Theta}{\sum\limits_{x = {t - u_{\Theta}}}^{t}{\Theta_{l}(z)}}} + {P_{\beta}{\beta_{l}(t)}} + {D_{\beta}\frac{d\;{\beta_{l}(t)}}{d\; t}} + {I_{\beta}{\sum\limits_{x = {t - u_{\beta}}}^{t}{\beta_{l}(z)}}}}},} & \left( {{Eq}.\mspace{14mu} 2} \right) \\{{{Q_{Rl}(t)} = {{P_{R}{R_{l}(t)}} + {D_{R}\frac{d\;{R_{l}(t)}}{d\; t}} + {I_{R}{\sum\limits_{x = {t - u_{R}}}^{t}{R_{l}(z)}}} + {P_{\lambda}{\lambda_{l}(t)}} + {D_{\lambda}\frac{d\;{\lambda_{l}(t)}}{d\; t}} + {I_{\lambda}{\sum\limits_{x = {t - u_{\lambda}}}^{t}{\lambda_{l}(z)}}} + {P_{\alpha}{\alpha_{l}(t)}} + {D_{\alpha}\frac{d\;{\alpha_{l}(t)}}{d\; t}} + {I_{\alpha}{\sum\limits_{x = {t - u_{\alpha}}}^{t}{\alpha_{l}(z)}}}}},} & \left( {{Eq}.\mspace{14mu} 3} \right) \\{{{Q_{Fl}(t)} = {{P_{F}{F_{l\;}(t)}} + {D_{F}\frac{d\;{F_{l}(t)}}{d\; t}} + {I_{F}{\sum\limits_{z = {t - u_{F}}}^{t}{F_{l}(z)}}}}},} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

where

β_(l)=the magnitude of the angle existing between the approximatelongitudinal axis of foot 1 and the projection of the approximatelongitudinal axis of the ipsilateral shank onto a plane substantiallyparallel to the sole of foot 1,

λ_(l)=the magnitude of the angle existing between the approximatelongitudinal axis of shank 1 and a plane substantially parallel to thesole of foot 1,

α_(l)=the magnitude of the angle existing between a line substantiallyparallel to the longitudinal axis of shank 1 and a line substantiallyparallel to the longitudinal axis of the ipsilateral femur,

z=a variable representing discrete time,

d/dt=the operator indicating a discrete-time estimation of the firstderivative,

Σ=the summation operator,

P_(θ), D_(θ), u_(θ), P_(β), D_(β), I_(β), u_(β), P_(R), D_(R), I_(R),u_(R), P_(λ), D_(λ), I_(λ), u_(λ), P_(α), D_(α), I_(α), u_(α), P_(F),D_(F), I_(F), u_(F) are time-invariant coefficients.

An example of an algorithm for the processing performed by software orfirmware stored in and executed by a microcontroller in the signalprocessing subsystem is as follows:

ALGORITHM:{grave over ( )} for v = 1 to V for p = 1 to P if {(A_(V) ≤Q^(J1)(t) < A_(V+1)) ∩ (B_(p−) ≤ Q_(θ1)(t) ≤ B_(p+))} = true, thenS_([v,p]1) (t) = h(Q_(K1) (t),Q_(θ1) (t)) else S_([v,p]1) (t) = 0 endend where

subscripts J and K correspond to subscripts F and R in Equations 3 & 4:(J=F and K=R) or (J=R and K=F),

[v,p] denotes the spatial coordinates of an individual stimulator withinthe stimulation array, v being an integer denoting the vertical indexand p being an integer denoting the horizontal index of the stimulatorat location [v,p],

∩ denotes the logical AND operation,

V=total number of horizontal rows in the stimulation array,

P=total number of vertical columns in the stimulation array,

A_(j)=the threshold of Q_(J1)(t) for activation of a stimulator in thejth row of the stimulator array,

B_(j−)=the minimum value of Q_(θ1)(t) for activation of a stimulator inthe jth column of the stimulator array,

B_(j+)=the maximum value of Q_(θ1)(t) for activation of a stimulator inthe jth column of the stimulator array,

h(Q_(K1)(t), Q_(θ1)(t)) is a piecewise-continuous function of Q_(K1)(t)and Q_(θ1)(t), and

s_([v,p]1) is the magnitude of activation of stimulator [v,p] within thestimulator array attached to leg 1. s_([v,p]1) may denote the amplitudeor frequency of stimulation produced by stimulator [v,p].

In a preferred embodiment, with regard to the stimulators, 3 vibratorrows are used to encode load, 4 vibrator columns are used to encodepolar center-of-pressure (COP) orientation, and vibrator activationvoltage (proportional to frequency) is used to encode polar radius ofCOP. Vibrator frequency is normalized over its active range to anelliptical ring with ranges of 1 to 8 mm in the mediolateral directionand 2 to 20 mm in the anteroposterior direction. These ranges wereselected based upon the typical range of human movement during quietstance and would be increased in order to optimize the device fordynamic activities such as walking. The position of force transducersunder the foot soles corresponds to a foot corresponding to a U.S. Men'ssize 9 shoe. Illustrative parameter values for an example of thepreferred embodiment are therefore provided for purposes of explanationas follows:

N = 7, r₁ = 75, r₂ = 80, r₃ = 90, r₄ = 120, r₅ = 70, r₆ = 80, r₇ = 115, (units  in  mm)θ₁ = 0.70, θ₂ = 1.05, θ₃ = 1.39, θ₄ = 1.74, θ₅ = 2.09, θ₆ = 4.36, θ₇ = 4.89, (units  in  radians)V = 3, P = 4, J ≡ F, K ≡ R, P_(θ) = P_(R) = P_(F) = 1; P_(β) = P_(λ) = P_(α) = 0;D_(θ) = D_(R) = D_(F) = D_(β) = D_(λ) = D_(α) = 0, I_(θ) = I_(R) = I_(F) = I_(β) = I_(λ) = I_(α) = 0, A₁ = 0.25  W, A₂ = 0.5  W, A₃ = 0.75  W, B¹⁻ = −0.25 π  rad, B₁₊ = 0.25π  rad, β_(2⁻) = 0.25π  rad, B₂₊ = 0.75π  rad, B³⁻ = 0.75π  rad, B₃₊ = 1.25π  rad, B_(4⁻) = 1.25π  rad, B₄₊ = 1.75π  rad$h \equiv {\left( {{Q_{Rl}(t)} - R_{O}} \right)\frac{V_{\max} - V_{\min}}{17}\mspace{14mu}{for}\mspace{14mu}{Q_{Rl}(t)}} > R_{O}$h ≡ 0  for  Q_(Rl)(t) ≤ R_(O) where$R_{O} = \sqrt{\frac{1}{{\cos^{2}\mspace{14mu}{Q_{\theta\; l}(t)}} + \frac{\sin^{2}\mspace{14mu}{Q_{\theta\; l}(t)}}{9}}}$V_(max) = 5  volts  and  V_(min) = 2.5  Volts

In an alternative embodiment, Equation 1 above also includes calculationof the total shear force impinging on the sole of foot 1, and Equation 2and/or 3 includes terms pertaining to the total shear force, thederivative of the total shear force, and the integral summation of thetotal shear force impinging on foot 1. Thus the representation ofbalance information by the stimulator array reflects informationregarding the shear forces impinging on the soles of the user's feet.

Neuropathic patients often encounter sustained elevated pressures underparts of their feet that result in skin damage and the development of anulcer. Consistent with the algorithm above, the disclosed system mayoperate such that signal processing subsystem transmits signals to thestimulators reflecting the time histories of pressures or forcesimpinging on individual transducers within a sensor array. The resultingstimulation to a wearer of such an embodiment indicates when and wherethe forces or pressures impinging on an individual transducer within thesensor array have exceeded a predetermined instantaneous or timeintegral threshold.

While in the above algorithm the terms foot, shank and femur are used todescribe body parts of a wearer of an embodiment of the disclosedsystem, those skilled in the art will recognize that other terms may beused in the alternative to describe the same parts. For example,alternative, corresponding terms to those used in the algorithm aboveinclude lower leg for shank, and upper leg for femur.

FIG. 4 shows an illustrative embodiment of the disclosed system using asingle level stimulation array 40. As shown in FIG. 4, the illustrativeembodiment includes a vibrotactile feedback array 40, a microprocessorcontroller subsystem 42, and a foot sole pressure sensor array 44. Inthe illustrative embodiment of FIG. 4, vibrotactile or electrotactilecutaneous feedback provided through the stimulation array 40 encodesposition of foot Center-Of-Pressure and/or weight distribution bymodulatingone or more of the following: stimulus frequency, stimulusamplitude, location of stimulus or number of active stimulators. Thisstimulation array 40 is located adjacent to the skin of the leg orthigh. The location of active stimulator(s) on the skin in thetransverse plane directly reflects the location of the footCenter-of-Pressure in the transverse plane.

The microprocessor controller subsystem 42 operates to convertelectrical or mechanical signal(s) from the sensor array 44 intosignal(s) which control the activity of elements within the feedbackarray 40. The microprocessor controller subsystem 42 may be implementedas a discrete system component or be imbedded within the othercomponents. The microprocessor controller subsystem

42 estimates the position of the Center-of-Pressure {COP) under the footand/or the fraction of the body weight supported by the foot. Theseestimates are then used to produce an appropriate output signal to thefeedback array 40. A “dead-zone” may be implemented such thatCenter-of-Pressure position within a certain range and/or foot loadbelow a certain threshold may produce no output to the feedbackelements.

FIG. 5 shows an illustrative embodiment of the disclosed system using amulti-level stimulation array. The illustrative embodiment of FIG. 5includes an array of force sensing resistors {FSRs) 50 placeable underthe soles of one or more feet of a user. Balance information from thearray of force sensing resistors 50 is passed to a microprocessor dataacquisition and processing subsystem 54. The subsystem 54 operates toconvert the balance information it receives into stimulation controlsignals sent to a vibro-tactile array 52 located on one or more legs ofthe user. The vibro-tactile array 52 includes three vibrator levels 56,58, and 59. A top view 60 of the vibrotactile array on the leg or legsof the user illustrates that each of the levels 56, 58 and 59 includefour vibrators located in the front, back, and both sides of the leg orlegs.

FIG. 6 shows an illustrative embodiment of the disclosed system using amulti-level stimulation array in combination with a bi-axial goniometer.As shown in FIG. 6, a force pressure sensor array 62 and a bi-axialgoniometer 64 operate to provide balance information to a microprocessordata acquisition and processing subsystem 66. The bi-axial goniometer 64provides information regarding detected ankle angle with regard to theangle of the user's foot to the corresponding lower leg. The stimulationarray 68 on the user leg is shown as a multilevel vibrator array, andthe top view 70 of the stimulation array 68 illustrates that each levelof vibrators in the stimulation array 68 includes four vibrators,mounted in the front, back, and both sides of the leg.

FIG. 7 illustrates an example of force magnitude encoding in amultilevel stimulator embodiment. FIG. 7 illustrates that a first,relatively low level force 72, may be encoded as a vibrational stimulus74 on a lowest vibrator layer (or level) of the stimulation array.Similarly, a relatively high level force 80, may be encoded as avibrational stimulus 82 on a highest vibrator layer (or level) of thestimulation array. Along these same lines, a force 76 between the forces72 and 80 may be encoded as a vibrational stimulus 78 within a middlevibrator layer of the stimulation array. Thus FIG. 7 shows how amagnitude represented in the balance information signal, such as aground reaction magnitude, can be communicated to the user throughlevels within a stimulator array attached or proximate to the user.

FIGS. 8-12 show the mapping of balance information to stimulation pointsin an illustrative embodiment, and thus illustrate mapping of COPinformation onto a body part, in this case, a leg of the user. FIG. 8shows a “dead zone” 86, representing a predetermined physical area undera user's foot. In an illustrative embodiment of the disclosed system, ifa center of pressure is estimated to be located within the dead zone, nostimulus is provided to the user. In such a case, the user issignificantly balanced on the foot in question. As shown in FIG. 8, inthe event that a center of pressure 88 is determined to be within thepredetermined dead zone, no vibrational stimulus is provided in any ofthe vibrators 90, 92, 94 or 96, shown within a single level ofstimulators mounted on a user's leg in a plane parallel to an array ofsensors mounted under the user's foot or feet.

FIG. 9 illustrates operation of an embodiment of the disclosed system inthe event that a center of pressure 98 is estimated to be locatedtowards the back of the user's foot. Under such circumstances, avibrator 96 located towards the back of the user's leg is shownproviding a vibrational stimulus to the user.

FIG. 10 illustrates operation of an embodiment of the disclosed systemin the event that a center of pressure 100 is estimated to be locatedtowards the front of a user's foot. Under such circumstances, a vibrator92 located towards the front of the user's leis shown providing avibrational stimulus to the user.

FIG. 11 illustrates operation of an embodiment of the disclosed systemin the event that a center of pressure 102 is estimated to be locatedtowards the front and right side of the user's foot. Under suchcircumstances, a vibrator 92 located towards the front of the user'sleg, and a vibrator 94 located towards the right of the user's leg areshown providing a vibrational stimulus to the user.

FIG. 12 illustrates frequency encoded magnitude stimulus in anillustrative embodiment. In the embodiment illustrated by FIG. 12, arelative increase in vibration frequency in one or more vibrators withinthe stimulation array is triggered by an increase in the polar radialdistance of the center of pressure from a coordinate frame located nearthe center of the foot, or from the edge of the dead zone.

As shown in FIG. 12, the radial distance of the center of pressure fromthe center of the user's foot may be encoded using the frequency ofvibration caused to occur in one or more vibrators in the stimulationarray. As shown in FIG. 12, an estimated center of pressure 104, locatedtowards the front of the user's foot, and being relatively farther fromthe center of the user's foot, may be represented to the user by causingthe vibrator 92 in the stimulation array to vibrate at a relativelyhigher frequency. Such a higher frequency vibration thus represents thefurther distance of the estimated center of pressure from the center ofthe foot to the user. For example, the distance of the estimated centerof pressure 104 from the center of the foot in FIG. 12 is greater thanthe distance of the estimated center of pressure 102 from the center ofthe foot in FIG. 11. Accordingly, the frequency of vibration of thevibrator 92 in FIG. 12 is greater than the frequency of vibration of thevibrators 92 and 94 in FIG. 11.

The disclosed system provides many and various advantages over previoussystems. Specifically, the simplification of the balance informationfeedback provided by the disclosed system can more easily be integratedinto the user's unconscious postural control system. The reduction ofindividual pressure signals by the disclosed system into an estimate ofCOP position and magnitude under each foot is easier to integrate intothe postural control system than information regarding a number ofseparate pressure transducers.

A further advantage of the disclosed system relates to the coding ofbalance information using frequency modulation in addition to or as analternative to amplitude modulation. Cutaneous stimulation has beenshown to excite cutaneous mechanoreceptors on a 1 to 1 basis for a widerange of input frequencies. As a result, some cutaneous mechanoreceptorswill respond to an artificial stimulation (vibrotactile orelectrotactile) in the same manner as they would respond to a pressurestimulus. Simulating a natural pressure stimulus with an artificial onein this manner should facilitate the integration of this informationinto the unconscious balance control system.

Moreover, the location of feedback stimulators on the legs and orientedin a plane parallel to the plane of the foot sole should facilitate theintegration of feedback information into the unconscious balance controlsystem.

A number of specific variations and modifications are foreseen withinthe scope of the present invention. The following are some examples ofvariations and modifications:

1. The sensor array and/or stimulation array may be incorporated into astocking, shoe, or boot.

2. The sensors may be embodied to acquire, encode, and provides feedbackregarding shear forces under the user's foot or feet.

3. The sensors may be embodied to acquire, encode, and provide balanceinformation regarding angle and or angular velocity of the lower legwith respect to the foot.

4. The disclosed system may be embodied to stimulate the cutaneous footsole for the purpose of reducing postural deficits associated withlong-term exposure to reduced foot loads, such as those incurred bybedridden patients on earth or astronauts in microgravity.

5. The disclosed system may be embodied to stimulate the cutaneous footsole for the purpose of producing an artificial feeling of pressure orshear force, such as might be used in virtual environments.

6. The disclosed system may be embodied to stimulate the skin of a partof the body other than the foot sole for the purpose of producing anartificial feeling of pressure or shear force, such as might be used invirtual environments.

7. The disclosed system may be embodied to stimulate the cutaneous footsole in response to pressure under the foot for the purpose ofamplifying the sensation of pressure.

8. The disclosed system may be embodied to implement a signal processingmethod such that a range of COP positions and/or magnitudes produce nooutput from the feedback array (i.e. sensory “dead zone”).

9. The mode of feedback may be embodied as tactile, vibrotactile,electrotactile, visual, thermal, and/or auditory.

10. The sensor array is implanted into or under the skin or within thebody.

11. The feedback array may be implanted into or under the skin or withinthe body.

12. The stimulation array may be implanted such that the feedbackelements are adjacent to or in contact with one or more sensory neuronsor sensory nerves.

13. The sensor array may be affixed to or embedded within a prostheticlimb.

14. The communication between any or all of the device components may bewireless.

15. The sensor signals and/or feedback signals may be monitored remotelyor recorded for the purpose of evaluating the effect or function of thedevice.

Those skilled in the art should appreciate that while the illustrativeembodiments may implement the functions of the signal processingsubsystem in computer software, these functions may alternatively beembodied in part or in whole using hardware components such asApplication Specific Integrated Circuits, Field Programmable GateArrays, or other hardware, or in some combination of hardware componentsand software components.

While the invention is described through the above exemplaryembodiments, it will be understood by those of ordinary skill in the artthat modification to and variation of the illustrated embodiments may bemade without departing from the inventive concepts herein disclosed.Accordingly, the invention should not be viewed as limited except by thescope and spirit of the appended claims.

What is claimed is:
 1. A system for improving balance control, thesystem comprising: a. at least one sensor positionable against a limb ofthe user, wherein the at least one sensor is configured to generate userbalance information signals and transmit the user balance informationsignals; b. a signal processing subsystem, the subsystem configured toreceive the user balance information signals and generate balancecontrol signals comprising temporal and spatial information reflectingan amount of force applied to the at least one sensor; and c. at leastone stimulator configured to be responsive to the balance controlsignals to provide stimulation when the amount of force exceeds thepredetermined force threshold, thereby providing feedback to the user.2. The system of claim 1, wherein the stimulation comprises vibratorystimulation, visual stimulation, auditory stimulation, electricalstimulation, electrocutaneous stimulation, or thermal stimulation. 3.The system of claim 1, wherein the force is normal force or shear force.4. The system of claim 1, wherein the user balance information signalscomprise time histories of force applied to the at least one sensor. 5.The system of claim 1, wherein the user balance information signalscomprise force or pressure as a function of time.
 6. The system of claim1, wherein the predetermined force threshold is a predetermined temporalor spatial threshold.
 7. The system of claim 6, wherein thepredetermined temporal or spatial threshold comprises an instantaneousor time integral threshold.
 8. The system of claim 1, wherein the userbalance information signals comprise information regarding the amount offorce applied to a sole of the user's foot.
 9. The system of claim 1,wherein the at least one sensor is sensitive to forces orientedperpendicular or parallel to the at least one sensor.
 10. The system ofclaim 1, wherein the at least one sensor is a foot pressure sensor. 11.A system for improving balance control, the system comprising: a. atleast one sensor positionable under a user's foot, the at least onesensor positioned to sense an amount of force applied to the at leastone sensor, wherein the at least one sensor is configured to generate,during user stance and dynamic activities, user balance informationsignals and transmit the user balance information signals; b. a signalprocessing subsystem, the subsystem configured to receive the userbalance information signals, compare the user balance informationsignals to a predetermined temporal or spatial force threshold, andgenerate, based on the user balance information signals and thepredetermined force threshold, balance control signals comprisingtemporal and spatial information reflecting the amount of force appliedto at least one sensor; and c. at least one stimulator configured to beresponsive to the balance control signals to provide stimulation duringthe user stance and dynamic activities when the amount of force exceedsthe predetermined force threshold, thereby providing feedback to theuser to allow corrective action to reduce or eliminate skin damage. 12.The system of claim 11, wherein the at least one stimulator isconfigured to provide vibratory stimulation, visual stimulation,auditory stimulation, electrical stimulation, electrocutaneousstimulation, or thermal stimulation.
 13. The system of claim 11, whereinthe force is normal force or shear force.
 14. The system of claim 11,wherein the user balance information signals comprise time histories offorce applied to the at least one sensor.
 15. The system of claim 11,wherein the user balance information signals comprise informationregarding the amount of force applied to a sole of the user's foot. 16.The system of claim 11, wherein the at least one sensor is sensitive toforces oriented perpendicular or parallel to the at least one sensor.17. The system of claim 11, wherein the at least one sensor is a footpressure sensor.
 18. A method of improving balance control, the methodcomprising: a. sensing, with at least one sensor, on a user's foot anamount of force applied to the at least one sensor, b. generating,during user stance and dynamic activities, user balance informationsignals as a function of the amount of force; c. transmitting the userbalance information signals to a signal processing subsystem; d.comparing, with the signal processing subsystem, the user balanceinformation signals to a predetermined temporal or spatial forcethreshold; e. generating, based on the user balance information signalsand the predetermined force threshold, balance control signalscomprising temporal and spatial information reflecting the amount offorce applied to the at least one sensor; and f. providing stimulationto the user during the user stance and dynamic activities based on thebalance control signals when the amount of force exceeds thepredetermined force threshold, thereby providing feedback to the user toallow corrective action to reduce or eliminate skin damage.
 19. Themethod of claim 18, wherein the stimulation comprises vibratorystimulation, visual stimulation, auditory stimulation, electricalstimulation, electrocutaneous stimulation, or thermal stimulation. 20.The method of claim 18, wherein the user balance information signalscomprise information regarding the amount of force applied to a sole ofthe user's foot.